libtraceevent: Fix header installation

When we passed some location in DESTDIR, install_headers called
do_install with DESTDIR as part of the second argument.

But do_install is again using '$(DESTDIR_SQ)$2', so as a result the
headers were installed in a location $DESTDIR/$DESTDIR.

In my testing I passed DESTDIR=/home/sudip/test and the headers were
installed in: /home/sudip/test/home/sudip/test/usr/include/traceevent.

Lets remove DESTDIR from the second argument of do_install so that the
headers are installed in the correct location.
Signed-off-by: Sudipm Mukherjee <sudipm.mukherjee@gmail.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Sudipm Mukherjee <sudipm.mukherjee@gmail.com>
Cc: linux-trace-devel@vger.kernel.org
Link: http://lore.kernel.org/lkml/20191114133719.309-1-sudipm.mukherjee@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/traceevent/Makefile

@@ -232,10 +232,10 @@ install_pkgconfig:
 
 install_headers:
 	$(call QUIET_INSTALL, headers) \
-		$(call do_install,event-parse.h,$(DESTDIR)$(includedir_SQ),644); \
-		$(call do_install,event-utils.h,$(DESTDIR)$(includedir_SQ),644); \
-		$(call do_install,trace-seq.h,$(DESTDIR)$(includedir_SQ),644); \
-		$(call do_install,kbuffer.h,$(DESTDIR)$(includedir_SQ),644)
+		$(call do_install,event-parse.h,$(includedir_SQ),644); \
+		$(call do_install,event-utils.h,$(includedir_SQ),644); \
+		$(call do_install,trace-seq.h,$(includedir_SQ),644); \
+		$(call do_install,kbuffer.h,$(includedir_SQ),644)
 
 install: install_lib